Disc drive memory systems have been used in computers for many years for storage of digital information. Information is recorded on concentric memory tracks of a magnetic disc medium, the actual information being stored in the form of magnetic transitions within the medium. The discs themselves are mounted on a hub that is rotatably mounted on a fixed spindle. The information is accessed by means of read/write heads generally located on a pivoting arm that moves radially over the surface of the disc. The read/write heads or transducers must be accurately aligned with the storage tracks on the disc to ensure proper reading and writing of information.
During operation, the discs are rotated at very high speeds within an enclosed housing by means of an electric motor generally located inside the hub that supports the discs. One type of motor in common use is known as an in-hub or in-spindle motor. Such in-spindle motors typically have a spindle mounted by means of ball or fluid dynamic bearing systems to a fixed motor shaft (spindle) disposed in the center of the hub. Generally, such motors include a stator formed in a base of the assembly, comprising a plurality of teeth arranged in a circle. Each of the teeth support coils or windings that may be sequentially energized to polarize the stator. A plurality of permanent magnets are disposed in alternating polarity on an inside rim of the hub, adjacent the stators. As the coils disposed on the stators are sequentially energized in alternating polarity, the magnetic attraction and repulsion of each stator to the adjacent magnets cause the hub to rotate, thereby rotating the disc and passing the information storage tracks across the head.
Motors used in disc storage systems can use hub and spindle assemblies having ball bearings, and/or fluid dynamic bearings. For both types of bearings, a lubricating fluid can be in a liquid or gaseous state. The use of fluid dynamic bearing assemblies in such drive systems has become preferred due to desirable reductions in drive size and noise generation as compared to conventional ball bearing drive systems. In fluid dynamic bearings, a lubricating fluid functions as the bearing surface between a spindle and a hub. Such bearings are of the journal and thrust types. Journal bearings fix the radial position of a hub as it rotates around a spindle. Thrust bearings constrain the axial position of the hub as it rotates.
As the demand for higher performance from drive systems increases, in terms of more revolutions per minute for a spinning disc, as well as lower power consumption for drive motors, the use of lower viscosity lubricating fluids and/or gasses in bearings becomes attractive to further reduce friction and rotational drag. It is well known that thin film head and disc interfaces are highly sensitive to damage from electrostatic discharge that can result from triboelectric charge build-up. Thus it is desirable for a conductive path to be established between a thin film head and a disc surface to prevent such charge build-up. For ball bearing based disk storage systems, the charge can get transferred either through the ball-raceway interface of the bearing or a ferrofluidic seal of the spindle motor. For fluid dynamic bearings where the fluid is a liquid, electrical charge can get transferred from the head-disc interface to the ground through the spindle motor fluid dynamic bearing, if the bearing gap is reasonably small. However if a the fluid is a gas, such charge transfer can be substantially blocked. Therefore, it is desirable to establish another conductive path between spindle and hub. Ferrofluid seals, wherein a ferrofluid containing metallic particle is positionally constrained by magnetic forces can be used, however such ferrofluids tend to outgas and otherwise contaminate a disc drive, degrading reliability. It is desirable to provide a conductive seal for a bearing assembly for a disc storage system that can prevent electrical charge buildup between the thin film head and the disc, that has a long operational life but does not suffer from the contamination and magnetic field confinement requirements of ferrofluidic seals.